It may not be a logical deduction, but to my imagination it is far more satisfactory to look at such instincts as… ants making slaves… larvae… feeding within live bodies… not as specially endowed or created instincts, but as small consequences of one general law, leading to the advancement of all organic beings, namely, multiply, vary, let the strongest live and the weakest die. (p.219-20)
As someone who has read his book The Origin of Species this quote makes perfect sense. I do believe that nature has the upper hand when dealing out the cards of life, and a species may have very little say in whether they survive or die off. If a bird is born without the ability to fly, and is born on an island with little sustenance, how is it capable of surviving? It can try and survive on what little it has, but when it is not able to adapt to its environment it will not survive. This goes for an entire species born without the proper variations to compete against its environment and other species or species of the same genus. For those species that do survive, their variations will continue through generations, and be combined with other variations until nature deals out one that is badly adapted. This species will then struggle to survive, but die if its competitors get the better of him. To me, this is the long, slow process of natural selection.
Darwin’s use of the word “strength” can be interpreted and applied in various ways. In my mind, it is important to include intelligence as a form of strength. Many individuals may have the intellectual strength to overcome their “opponents” in the struggle for life through taking advantage of them, conning them or depending on them. In this way though, Darwin may also include “habit” as a form of intelligence, only an intelligence built in to a species subconscious. In all, Darwin’s theory that the “strongest live and the weakest die” rings true in the laws of nature.